How to effectively manage preschoolers?

I'm a swim instructor at a local pool. recently, I was doing a placement test on a 3 year old girl. I couldn't get her to focus for more than literally 5 or 6 seconds and she continually tried to break free from me to splash around in the water. Normally, i would let her do this for a minute or two as long as she was being safe about it, but we were on a very tight schedule and i needed to get her in the pool and back out again. Eventually I came to the decision that she should be in level 1, and then I asked her to get out of the pool. I ended up walking hand in hand with her up the ramp because she wouldn't listen. I used all sorts of strategies to get her out... counting to three, telling her that it's time to go see mommy, telling her that play time was over and it was time to get out, etc... It ended up being necessary for me to hold her firmly by the hand and drag her out of the pool. Only problem, she kept breaking free and running down the ramp away from me and I had to start all over. I eventually called my supervisor over and he tried all the same things, with no luck. Finally, her dad came over and plucked her out of the pool. This little girl has a twin, too, and whenever they saw each other, chaos ensued. Parents always sign their kids up for the same lessons, and if I get these two girls together, I'd be completely at a loss as to how to deal with these two, as well as 3 or 4 other preschoolers. Any tips for effectively managing preschoolers so I can avoid another encounter like this?